Practical considerations in raising an experimental mouse colony are dealt with in an article entitled "A Flexible Barrier at Cage Level for Existing Colonies: Production and Maintenance of a Limited Stable Anaerobic Flora in a Closed Inbred Mouse Colony" by R. S. Sedlacek, R. P. Orcutt, H. D. Suit and E. F. Rose, published in Recent Advances in Germfree Research, pp. 65-69, Tokai University Press, 1981. The mentioned article describes a standard mouse shoe box made of transparent plastic, e.g., polycarbonate, and also describes what is referred to as a filter bonnet for the shoe box or cage. The bottom edges of the filter bonnet rests on the top edges of the side walls of the mouse box. The open top of the filter bonnet is fitted with a replaceable filter sheet containing a Remay 20-24 filter medium protected by a perforated aluminum plate. Since the filter bonnet is designed to fit tightly over the animal cage, the system acts as a static system and prevents particulate material from entering the cage. This type of cage and caging system has been made and sold as the Micro-Isolator System by Lab Products, Inc., 255 West Spring Valley Avenue, Maywood, N.J. 07607 and the trade literature of this company describes the cage in more detail.
While the mentioned prior art caging system has been shown to be effective in maintaining animals for up to thirty months with a constant microbial flora, i.e., free of exogenous microbial contamination, the system has a major disadvantage in that air exchange is predicated upon gradient diffusion. Gases of greater concentration diffuse through the filter in the direction of lesser concentration. Thus, under these conditions CO.sub.2 and NH.sub.3 from the animals will accumulate and will only diffuse slowly and outwardly from the cage and O.sub.2 and N will diffuse slowly into the cage. As a result of this disadvantage, the population within the cage must be kept relatively low and/or the cage must be changed every three to four days.
With the foregoing background, the present invention has as its object providing a still-further improved cage and caging system and method which eliminates the disadvantage of the air exchange being predicated solely upon gradient diffusion. The present invention also has as an object providing an improved cage, caging system and method which allows the static system to resume operation when removing the invention cage for operations such as animal manipulation and/or cage change such as would be carried out in a laminar flow cabinet.